measure of dispersion standard deviation Orlaith Burke Alejandra Gonzalez-Beltran Philippe Rocca-Serra The standard deviation of a random variable, statistical population, data set, or probability distribution is a measure of variation which correspond to the average distance from the mean of the data set to any given point of that dataset. It also corresponds to the square root of its variance. http://en.wikipedia.org/wiki/Standard_deviation sd(x, na.rm = FALSE) http://stat.ethz.ch/R-manual/R-patched/library/stats/html/sd.html σ ready for release